Uneven sedimentation in laboratory centrifuges during experiments.

Release time:

2012-03-05

Overview:

Laboratory centrifuges are widely used in laboratories and serve as an indispensable and powerful tool in biological research techniques involving biomacromolecules, organic macromolecules, subcellular components, proteins, nucleic acids, and more.

The speed difference between the rotor and the inner screw of the laboratory centrifuge is 28 rpm. The inner-screw bearings are lubricated with grease, which needs to be replaced every quarter. However, during the replacement in April 2003, after a large amount of grease was added from one side, no new grease could be forced out from the other side—this was an unusual situation. Upon disassembling and inspecting the centrifuge, we found that water had entered the inner-screw bearings and the mechanical seal had been damaged, causing the grease to leak out through the seal. The maintenance was carried out relatively promptly, thus preventing the bearings from burning out.
　　
　　However, each time the resulting precipitate is not entirely homogeneous. This is because particles of varying sizes and densities are evenly distributed throughout the centrifuge tube. After a certain period, once all the heaviest particles have settled to the bottom of the tube, even the lighter particles—those initially closer to the bottom of the new tube—begin to settle as well, mixing with the heavier particles. Therefore, it’s necessary to resuspend the precipitate and re-centrifuge it at the original speed. The precipitate obtained after this second centrifugation will be somewhat purer. By repeating this process several times, we can eventually obtain a relatively homogeneous and highly pure sample. We refer to this procedure as “washing the precipitate.” After each wash, the supernatant is collected and combined, then subjected to another round of centrifugation at a higher speed. From either the supernatant or the precipitate, we can isolate and purify another distinct fraction. Differential centrifugation is commonly used to separate various subcellular components as well as crude extracts of nucleic acids and proteins. For example, tissue homogenates can be fractionated into different groups based on their sedimentation coefficients (s-values). Using high-speed centrifugation in this manner, the initial repeatedly resuspended precipitate can be subjected to two or three rounds of centrifugation, yielding a relatively pure subcellular fraction. Nevertheless, even after this process, the resulting fraction is still not completely homogeneous. To achieve an even better separation, one can further replace the uniform suspension medium with a continuous liquid density gradient.
　　
　　Laboratory centrifuges are widely used in laboratories and serve as an indispensable and powerful tool in biological research techniques involving biomacromolecules, organic macromolecules, subcellular components, proteins, nucleic acids, and more.

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